CHAPTER 4 — VARIABLES & DATA TYPES

CHAPTER 4 — VARIABLES & DATA TYPES - Printable Version

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CHAPTER 4 — VARIABLES & DATA TYPES - Leejohnston - 11-15-2025

Chapter 4 — Variables & Data Types
Learning how computers remember information

Up to now, Python has only printed text.

But real programs need to:
• store information
• remember things
• update values
• perform calculations
• keep track of data

For that, we use variables.

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4.1 What Is a Variable?

A variable is like a labelled box in the computer’s memory.

You choose the label,
you put something in the box,
and Python remembers it.

Example:

Code:
name = "Alice"

age = 15

• The variable name stores the text “Alice”
• The variable age stores the number 15

Whenever you use the variable later, Python looks inside the box.

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4.2 Creating Variables (The Simple Rule)

A variable is always created like this:

Code:
variable_name = value

NOTE:
• no spaces around the variable name
• one equals sign (=)
• Python reads it as “store this value here”

Examples:

Code:
city = "London"

score = 92

price = 4.99

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4.3 Data Types — What Kind of Information?

There are 4 beginner-friendly data types:

1. String — text

Code:
name = "Johno"

2. Integer — whole number

Code:
age = 39

3. Float — decimal number

Code:
temperature = 18.7

4. Boolean — True or False

Code:
is_online = True

These are the building blocks of all beginner programs.

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4.4 Printing Variables

You can print the value inside a variable:

Code:
name = "Mia"

print(name)

Output:

Code:
Mia

You can combine text + variables:

Code:
age = 16

print("Age:", age)

Output:

Code:
Age: 16

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4.5 Changing a Variable

Variables can be updated at any time:

Code:
score = 10

print(score)

score = 15

print(score)

Output:

Code:
10

15

Python always uses the most recent value.

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4.6 Naming Variables (Important Rules)

Variable names:
• must start with a letter
• cannot start with a number
• cannot contain spaces
• must not use special symbols (!, ?, /, %)
• can use underscores ( _ )

Examples:

Valid:

Code:
user_name

player2

temperature_reading

Invalid:

Code:
2score

user-name

my variable

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4.7 Combining Variables

Python can use variables in calculations.

Example:

Code:
a = 4

b = 6

total = a + b

print(total)

Output:

Code:
10

Another example:

Code:
length = 12.5

width = 4

area = length * width

print(area)

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4.8 Strings vs Numbers

Very important difference:

Code:
age = 20      # number

age_text = "20"  # text

One is a number you can calculate with.
The other is just characters.

If you add a number + text, Python gets confused:

Code:
age = 20

print("Age: " + age)    # ERROR

You must convert the number:

Code:
print("Age: " + str(age))    # Works

You’ll learn more conversions later.

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4.9 Exercise — Create a Personal Profile Program

In a file called profile.py, store these variables:

• your name (string)
• your age (integer)
• your favourite food (string)
• whether you like coding yet (boolean)
• your lucky number (integer)

Then print them all nicely, e.g.:

Code:
print("Name:", name)

print("Age:", age)

print("Favourite food:", food)

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4.10 Small Challenge

Create a program that stores:
• a product price
• the tax percentage
• the final price after tax

Then print the final result.

Example formula:

Code:
final_price = price + (price * tax_rate)

This is your first step towards real-world applications.

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4.11 Chapter Summary

• A variable stores information
• Strings = text, Integers = whole numbers, Floats = decimals, Booleans = True/False
• Variables can be printed, updated, and used in calculations
• Naming rules help keep your code clean
• You now understand the memory system behind all programs

Next chapter:
Chapter 5 — Input & Output

This is where your programs become interactive.

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Written and Compiled by Lee Johnston — Founder of The Lumin Archive